1. Field of the Invention
The present invention relates to hydraulic devices which are operated within a full case of hydraulic fluid. More specifically, the present invention relates to a hydraulic pump and hydraulic motor combination of a constant speed drive transmission which is operated in a case full of hydraulic fluid.
2. Description of the Prior Art
FIG. 1 illustrates a portion of a prior art constant speed drive 10 marketed by the assignee of the present invention. As is known, a constant speed drive transmission converts an input variable rotational shaft velocity from an input shaft coupled to an engine, such as an aircraft propulsion engine, to a constant rotational velocity shaft velocity on an output shaft. The output shaft drives a three-phase alternator (not illustrated) for producing constant frequency alternating current such as 400 Hz used in airframes. The illustrated portion 10 of the constant speed drive is known as a "full case" unit which has a case 12 in which is located a conventional hydraulic pump and hydraulic motor. The shaft 18 drives a conventional variable displacement hydraulic pump. The hydraulic pump functions to receive hydraulic oil from a charge pump 26, further pressurize the oil and apply the pressurized oil to the hydraulic motor. The charge pump 26 is illustrated schematically as being inside the case 12 but it should be understood that in practice the charge pump is external to the case with a conduit coupling oil from within the case to the charge pump outside the case with a conduit coupling the pressurized oil from the pump to the interior to the case to filter 30. The pressurized hydraulic fluid from the hydraulic pump drives a fixed displacement hydraulic motor. A control valve modulates the angle of a swash plate in the variable displacement hydraulic pump to vary pump displacement which controls the rotational velocity of the shaft 20. The conventional hydraulic pump and motor 14 are totally immersed within hydraulic fluid 16 during operation. The pump has an input port 22 which receives hydraulic fluid 16 which is pumped from the interior of the case 12. Hydraulic fluid is supplied to the input port 22 by charge pump 26 which receives oil from fluid line 28 which has an input port 29 in fluid communication with the hydraulic fluid 16 within the case 12. The opening of the input port in an intermediate position within the case 16 provides for fluid to be available to the hydraulic pump and motor 14 upon commencement of their operation. Pressurized oil which is outputted from the charge pump 26 is coupled to filter 30 by fluid line 32. The filter 30 is necessary to prevent any debris taken into the port 29 from the hydraulic fluid 16 from being pumped into the hydraulic pump and motor 14 which could cause substantial wear or damage. A charge relief valve 34 has a preset pressure which regulates the operating pressure of the hydraulic pump and motor 14 and discharges excess hydraulic fluid back into the full case 16. The charge relief valve 34 opens when the system operating pressure at the input port 22 rises above the threshold pressure of the valve to limit the system operating pressure. The charge relief valve 34 is connected to the input port 22 by fluid line 36. Leakage from the hydraulic pump and motor 14 flows back into the case to help contain fluid within the case along with fluid from the charge relief valve also within the case 12.
The contained external hydraulic fluid circuit 38 is described as follows. Hydraulic fluid circulates between outlet port 40 of the case 12 and inlet port 62 by flow through fluid line 44 into sump 46 where the hydraulic fluid 48 within the sump 46 is taken up by fluid line 49, pumped by scavenge pump 50 through fluid line 52 to scavenge filter 54, through fluid line 56 to heat exchanger 58 where the hydraulic fluid is cooled as the primary heat regulating mechanism for the hydraulic fluid 16 within the case 12 and finally flows through fluid line 60 to inlet port 62. Outlet port check valve 64 prevents the flow of hydraulic fluid from the case 12 in circumstances when the level of fluid in the case 12 is low. Inlet port check valve 66 also prevents the flow of hydraulic fluid 16 from the sump 12 back to the heat exchanger 58. The inlet port valve is spring biased into a closed position to prevent opening until a predetermined pressure is reached. However, the valve 66 does not prevent aspiration of gas by the charge pump 26.
The prior art full case unit 10 of FIG. 1 has several disadvantages. First, the hydraulic fluid supplied to the hydraulic pump and motor 14 is supplied generally from the hydraulic fluid 16 within the case 12. Because of the heat outputted by the hydraulic pump and motor 14 to the hydraulic fluid within the case 12, the temperature of the hydraulic fluid which is pumped by the charge pump 26 to the input port 22 of the hydraulic pump is at an elevated temperature which is the ambient temperature of the hydraulic fluid within the case. An elevated hydraulic fluid temperature reduces the service life of the hydraulic pump and motor 14 over that which would be achieved if the temperature of the hydraulic fluid inputted by the input port 22 were reduced. Second, as has been pointed out above, debris accumulates within the case 12 which if pumped into the hydraulic pump and motor 14 by the charge pump 26 could cause wear or premature failure which necessitates the use of a filter 30. The filter must be periodically replaced and further represents a weight penalty which in the preferred application of the present invention in the electrical power supply of an airframe is a disadvantage. Third, the rate of flow of hydraulic fluid through the external hydraulic fluid circuit including heat exchanger 58 must be sufficiently great to regulate the temperature of the oil within the full case 12. This necessitates the introduction of a sufficiently large quantity of cooled hydraulic fluid into the case 12 to bring down the temperature of the full case 12. The requisite flow rate to achieve the aforementioned cooling necessitates the use of a scavenge pump 50 having a relatively large flow rate with a concomitant weight penalty. The case 12 can be subject to leakage which lowers the level of hydraulic fluid in the case so that an interruption of the flow of fluid to the hydraulic pump and motor 14 is possible upon commencement of operation of the hydraulic pump and motor after long periods of inactivity. Any loss of hydraulic fluid to the hydraulic pump and motor 14 during initiation of operation can cause substantial damage.